The invention relates to a method as well as an arrangement for determining cylinder-individual differences of a control variable in a multi-cylinder internal combustion engine.
Ever higher requirements are imposed on modern internal combustion engines, for example, with respect to the smooth running and a reduction of the toxic substance emission. These requirements should be satisfied for all load conditions of the engine. The various load conditions result essentially from the actual operating situation while considering a requirement for the reduction or increase of the torque made available by the engine. A primary task of the control for the internal combustion engine is to adjust the torque generated thereby and for this purpose, in different component systems of the control, variables, which influence the torque, are controlled.
A central control variable of modern engine controls is the so-called charge which is influenced in the component system xe2x80x9ccharge controlxe2x80x9d. In the sense of this application, the term xe2x80x9cchargexe2x80x9d means essentially the mass of unconsumed oxygen which is available for the combustion. The charge is also characterized as air charge. In addition to the charge control, a precise control of the mixture composition is required, that is, the fuel enrichment in the air/fuel mixture. The air/fuel mixture ratio is characterized by the air ratio xcex which indicates the ratio between the supplied air quantity, which determines the charge, and the theoretical air requirement for complete combustion. Accordingly, xcex=1 corresponds to an ideal value with the view to an optimal, residual-free combustion; whereas, values xcex less than 1 correspond to an air deficiency or a rich mixture and values xcex greater than 1 correspond to an air excess or a lean mixture. In the component system xe2x80x9cmixture formationxe2x80x9d of the control, the fuel mass, which corresponds to a charge, is computed and the required injection time and the optimal injection time point are determined therefrom. Finally, a properly timed ignition of the mixture affects the course of the combustion.
Various suggestions have already been made to carry out an optimized engine control for internal combustion engines having cylinder-individual fuel metering, especially for engines having direct injection of fuel into the combustion chambers of the individual cylinders. Accordingly, it is, for example, known that manufacturing tolerances in the manufacture of injection valves lead to cylinder-individual mixture differences in operation especially for spark-ignition engines having fuel direct injection. These mixture differences become manifest in different torque contributions of the individual cylinders to the total torque of the engine. This can lead to rough engine running. This problem can be solved by adapting or equalizing the cylinder-individual torque contributions. Individual contributions of the individual cylinders to the rough running are determined via suitable measures and the cylinder-individual torque contributions are so controlled that the cylinder-individual rough running values approach a common desired value. An example of such an engine control is described in U.S. Pat. No. 4,688,535. Another example is disclosed in DE 198 28 279.
It has already also been suggested to improve the engine control with respect to the optimization of the toxic substance discharge. For this purpose, a cylinder-individual lambda control is discussed, for example, in the publication xe2x80x9cDevelopment of the High Performance L4 Engine ULEV Systemxe2x80x9d of N. Kishi et al in SAE 980415, starting at page 27. This cylinder-individual lambda control is intended to control the lambda ratio value xcex of the individual cylinders to the same optimal value for each cylinder in order to make possible a toxic substance optimized combustion individually for each cylinder.
A method according to the invention as well as an arrangement according to the invention for determining cylinder-individual differences of a control quantity in a multi-cylinder internal combustion engine are characterized in that cylinder-individual charge differences are determined. The invention thereby permits an access to a command variable, namely, the charge of the individual cylinders compared to the charge of the other cylinders. The command variable essentially codetermines the combustion and the operation of the engine. The possibility presented by the invention of recognizing or determining cylinder-individual charge differences permits a consideration of the charge differences when computing other cylinder-individual engine variables as well as, if required, a correction of cylinder-individual charge differences for their equalization. These engine variables are essentially influenced by the charge. Conventionally only fuel-metering differences had been corrected.
The invention permits also a reliable differentiation as to whether occurring torque differences between individual cylinders are caused essentially by cylinder-individual differences in the charge or cylinder-individual differences in the air ratio xcex which, as is known, is dependent upon the charge as well as on the corresponding fuel metering.
The result of a determination of cylinder-individual charge differences can, for example, be utilized in order to optimize the ignition angle. The term xe2x80x9cignition anglexe2x80x9d relates to the angular position of the ignition time point to a reference point, for example, to the top dead center point of the piston of a cylinder in its combustion stroke. In this way, improvements can be achieved compared to conventional knock controls. If an engine has the possibility of a cylinder-individual control of the air metering (for example, cylinder-individual throttle flaps), then determined cylinder-individual charge differences can, in accordance with the invention, be utilized directly to control this throttle flap and therefore to equalize the cylinder charges. This is especially advantageous for asymmetric engine geometries wherein, because of construction (for example, because of different dimensioning of the induction lines for the individual cylinders), high cylinder-individual charge differences can occur. The cylinder-individual charge differences can, however, also be utilized to optimize the injection time points.
A preferred embodiment provides that the determination of the cylinder-individual charge differences can be carried out with an equation which contains, as a variable, the cylinder-individual air ratio xcex and cylinder-individual torque contributions. In this way, it is possible with the determination of cylinder-individual charge differences, to omit a measurement of the individual cylinder charges so that installing corresponding sensors is not necessary. Rather, it is sufficient when suitable units are provided for the cylinder-individual torque detection or determination and for cylinder-individual detection or determination of the air ratio and when corresponding signals of these units are suitably combined. This simple computation of cylinder-individual charge differences is based, inter alia, on the recognition that essentially two influence quantities are decisive for a torque outputted by a cylinder, namely, on the one hand, the mentioned charge, that is, the oxygen mass which is available for the combustion and, on the other hand, the mixture composition indicated by the xcex value in which, in addition to the mass of combustion oxygen, also the supplied fuel mass is included. A comparatively slight dependence of the torque on the ignition angle which may possibly be given is not considered in this first approximation. These contributions are negligible especially in stratified operation of the engine.
A preferred further embodiment is characterized in that an equalization of the cylinder-individual air mass is carried out via a suitable equalization control and that, thereupon, cylinder-individual torque differences can be determined by direct measurement or indirect derivation and that the cylinder-individual charge differences are derived from the cylinder-individual torque differences. Here, it is assumed that, for the same xcex values in all cylinders, possibly present differences in the torque outputs of the individual cylinders are caused by different charges of the corresponding cylinders. Especially, it is assumed that there is a direct proportionality between cylinder-individual torque and cylinder-individual charge. The cylinder-individual xcex control is advantageously carried out to a value xcex=1 with a view to a minimum toxic substance discharge.
The determination of cylinder-individual torque differences required for this variation can be carried out in any possible way, for example, in that the torque contributions of individual cylinders to the total torque or variables, which are in direct relationship to these contributions, are measured by suitable sensors. For example, suitable combustion pressure sensors or torque sensors can be provided. Such expensive measurements can be omitted when the determination of cylinder-individual torque differences is carried out based on an evaluation of the rough running of the engine. All suitable methods and/or apparatus can be utilized for this purpose, for example, the method for cylinder equalization described in U.S. Pat. No. 4,688,535 wherein segment times are detected to evaluate the time-dependent trace of the rotational movement of the crankshaft or camshaft and, from this, an index for the rough running of the engine is formed. An improvement of this method is disclosed in DE 198 28 279. It is also possible to utilize rough running values which are anyway provided for the control for a possibly present combustion misfire detection. The formation of rough running values for combustion misfire detection is, for example, known from U.S. Pat. No. 5,861,553. The features of these publications, which relate to the formation of rough running values and the derivation of cylinder-individual torque contributions, are expressly included in this application.
It can also be that the cylinder-individual torque contributions are equalized by a corresponding control, for example, by cylinder-specific injection times and that, thereupon, cylinder-individual differences of the xcex value are determined. For controls having the possibility of the above-mentioned equalization of the xcex values, corresponding cylinder-individual lambda values are anyway present as input values of a control. Alternatively, or in addition, separate measuring devices can also be provided (for example, individual lambda probes) for measuring the individual lambda values. The cylinder-individual charge differences can then be derived from the cylinder-individual differences of the air ratio (for essentially equal torque contributions of the individual cylinders). This derivation is based on the assumption that, at least in a first approximation, the supplied fuel mass (contained in the air ratio xcex) is directly proportional to the outputted torque of the cylinder.